This invention relates to adjusting clocks in electronic devices to avoid collisions between undesirable clock frequency harmonics and signal frequencies that are being used by radio-frequency (RF) transceivers or other device components.
Electronic devices are often provided with wireless communications capabilities. For example, handheld electronic devices may use long-range wireless communications to communicate with wireless base stations. Handheld electronic devices such as cellular telephones may communicate using cellular telephone bands at 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. Electronic devices may also use short-range wireless communications links. For example, electronic devices may communicate in unlicensed bands using the IEEE 802.11 standard (2.4 GHz and 5 GHz) or Bluetooth® (2.4 GHz). Communications are also possible in data service bands such as the 3G data communications band at 2100 MHz that is used by the Universal Mobile Telecommunications System (UMTS). Devices with Global Positioning System (GPS) capabilities receive GPS signals at frequencies such as 1575 MHz.
Devices that operate in cellular telephone and wireless data communications bands include radio-frequency transceiver circuits. These circuits, which are sometimes referred to as radios, may be used to handle transmitted and received signals in one or more radio-frequency bands of interest.
Electronic devices also have other components such as displays, processors, and memory. Clock circuits are used to distribute a common time reference to these components. For example, a crystal oscillator may be used to generate a reference clock signal. Clock circuitry may be used to create clock signals such as square waves from the output of the crystal oscillator. For example, a phase-locked loop circuit may be used to create a clock signal at a multiple of the crystal oscillator's frequency.
A clock that operates at a given frequency f may produce signals at harmonic frequencies (e.g., fundamental harmonic f and higher order harmonics 2f, 3f, 4f, 5f, etc.). In a given electronic device, these harmonic frequencies may overlap with the frequencies of other signals in the device such as the frequencies used by radio-frequency transceiver circuitry. If care is not taken to properly isolate these overlapping signals, the device may not operate properly.
For example, the Digital Visual Interface (DVI) protocol is a display interface that is commonly used to drive digital displays such as liquid crystal display (LCD) monitors. In DVI signals, red, green, and blue pixel data and a clock are differentially encoded. For a standard timing, an LCD monitor with a 1920×1200 pixel resolution that operates at 60 Hz with a reduced blanking interval has a pixel clock that operates at 154 MHz. The 16th harmonic of this signal is centered around 2.464 GHz, which is close to the center frequency of IEEE 802.11 channel 11 at 2.462 GHz. If the DVI clock at 154 MHz is not well isolated from the radio-frequency transceiver used for the IEEE 802.11 signals, the 16th harmonic of the DVI clock will collide with IEEE 802.11 signals on channel 11. Adjacent channels may also be affected. As a result of the signal collision, a user may experience reduced throughput at a given range or communications using the desired frequency (channel 11 in this example) may not be possible.
As a result of the potential for undesirable signal collisions, extensive consideration is given to proper electromagnetic shielding in modern electronic devices. This typically entails providing additional electronic components in a device whose purpose is to reduce the impact of signal collisions. For example, certain components may be electromagnetically shielded by mounting the components within conductive enclosures. Signal interference can also be minimized by using filter networks.
These schemes generally help to reduce signal collisions between clock sources and component operating frequencies. Nevertheless, there can be severe penalties associated with shielding schemes. Metal enclosures consume valuable space and add cost and complexity to a device. Particularly in small-form-factor devices, there may be insufficient space for a conductive enclosure. Filtering components may add undesirable cost to a design and must be carefully selected to avoid adversely affecting device reliability.
It would therefore be desirable to provide ways in which to reduce the adverse impact of potential signal collisions in electronic devices with wireless communications circuitry.